President Barack Obama has adopted a cautious approach to the ceasefire announced Wednesday between Ukrainian President Petro Poroshenko and Russian President Vladimir Putin.

“It’s too early to tell what this cease-fire means,” said Obama, explaining that he had not seen full details but “only wire reports.”

News of the ceasefire broke shortly after Obama’s arrival in Estonia, where he is showing support to a Baltic ally before traveling to a crucial NATO summit in the U.K. at which the Ukrainian situation feature high on the agenda.

Obama promised to address the Ukrainian ceasefire more fully during a speech later in the day. In the meantime, he noted that Russia’s forceful incursion into Ukraine served as a stark reminder of the importance of the NATO alliance.

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“Obviously what’s happened in Ukraine is tragic but I do think it gives us an opportunity to look with fresh eyes and understand what it is that’s necessary to make sure that our NATO commitments are met. And that’s one of the reasons I’m here in Estonia today.”

Estonian President Toomas Hendrik Ilves said “I just did hear that President Poroshenko and President Putin have agreed on a ceasefire. I just hope it holds.”

Speaking ahead of Obama’s arrival, at a ceremony marking the 20th anniversary of the withdrawal of Russian troops from Estonia, Ilves said that the situation in Ukraine showed “that the principle of collective territorial defense hasn’t gone away,” according to the AP.

His remarks came a day after Mikhail Popov, a Kremlin security adviser, stated in Moscow that NATO remained Russia’s biggest threat.